Diffusion MR imaging is commonly used in clinical practice for early detection of brain injury from a variety of causes. Image contrast for this MR method is based on microscopic, incoherent water displacements. Within minutes of brain injury, water displacements (measured as the "apparent diffusion coefficient" or ADC) are reduced. Remarkably, the underlying biophysical basis for this decrease in ADC is not yet understood, though it appears related primarily to a reduction of the displacements of water in the intracellular space. We propose to better characterize the cellular-level changes in water displacement that accompany injury and allow detection of injury by diffusion-based MR imaging. A clearer understanding of the basis for diffusion contrast associated with cell injury will lead to more effective use of this MR method in the clinic and as a research tool. These studies will also provide values for intracellular water exchange lifetimes and cell membrane permeabilities for various cell types. This information will be useful for MR experiments in which tissue modeling of tissue water compartmentation is necessary. Our approach involves two model systems tissue culture and intact rat brain. With regards to tissue culture, there is indirect evidence that the response of glial cells to injury differs from that of neurons, with glial cells showing a larger change in intracellular water ADC. If this is the case, then diffusion imaging may be relatively insensitive to injury to neurons per se. We will test this hypothesis by measuring the response of intracellular water ADC to injury for neurons, glia, and skeletal muscle cells in culture. We will also use the data obtained from these cells, in association with their geometry as measured through microscopy, to develop models of the behavior of intracellular water. Studies of intact brain will involve diffusion measurements of reporter molecules confined to the intracellular space. These molecules will be cesium (a potassium analog which will be present in all cells) and N- acetylaspartate (NAA, present only in neurons). Since these molecules are present in the aqueous phase of cytoplasm, changes in their ADC values reflect changes in that of the intracellular water in which they are hydrated. Data from cesium and NAA before and after global ischemia will be evaluated in terms of the information obtained from tissue culture systems, including models specific for the geometries of neurons and glia. Finally, studies of intact rat brain will include measurement of intracellular water diffusion characteristics. This will be done by separating signal from intra- and extracellular water on the basis of differences in T1 relaxation following intracerebroventricular infusion of a relaxation agent. The models developed in the studies above will be applied to these data to determine the changes in intracellular viscosity associated with brain injury. PUBLIC HEALTH RELEVANCE: This study is designed to improve our understanding of how diffusion MR images detect brain injury. This is important because diffusion imaging will likely be used in the near future to identify people who would benefit from treatments to reduce brain injury after events such as a stroke. A better understanding of how this imaging works would enable us to use it more rationally and effectively.